In the eye (as well as all other body tissues except the lungs), arteries deliver blood to a tissue and veins remove blood from a tissue. The arteries carry oxygen and nutrition to the tissue, while veins remove waste products. A tiny retinal vein can become blocked by a blood clot; this blockage creates a backup in the system, resulting in leakage of blood and plasma (fluid from the bloodstream) into the retinal tissue. The retinal bleeding (hemorrhage) and swelling (edema) can cause significant visual loss. In addition, disruption of the blood supply can cause permanent damage to the retinal tissue from lack of oxygen and nutrition. In some cases, this may result in the development of abnormal blood vessels as a response to the lack of oxygen.
This condition typically occurs in individuals greater than 50 years old. Risk factors include hypertension, cardiovascular disease, diabetes, and glaucoma. Sometimes, individuals with retinal vein occlusion are found to have blood clotting disorders or inflammatory conditions. The second eye is affected in approximately 10% of cases.
Clinical Appearance & Symptoms
Retinal vein occlusions are classified as central retinal vein occlusion (CRVO) if the blockage occurs in the main vein leaving the eye through the optic nerve (Figure 1, 2). They are classified as branch retinal vein occlusion (BRVO) if the blockage occurs at one of the branches before reaching the main vein at the nerve; these often occur where arteries and veins cross (Figure 3, 4).
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When patients have a retinal vein occlusion, they frequently complain of blurred vision, distortion, and/or a central blind spot.
Occluded veins typically appear tortuous, just as a hose tends to twist when water flow is blocked. There is bleeding seen in the distribution of the blocked vein in BRVO; the whole retina is involved in CRVO. There is also usually swelling of the retina in the same distribution; macular edema is a common finding and is often a source of significant visual loss.
When there is a severe blockage in which blood flow is cut off or significantly reduced, sensitive retinal tissue can become permanently damaged. This process is called ischemia. As a response to ischemia, the eye may develop abnormal new blood vessels called neovascularization. Unfortunately, these new vessels are fragile and tend to bleed, potentially resulting in vitreous hemorrhage. They may also scar and pull on the retina, potentially resulting in traction retinal detachment. Finally, if these blood vessels grow in the drainage structure of the eye, a very serious form of glaucoma can develop called neovascular glaucoma.
When a retinal vein occlusion is suspected, certain diagnostic tests may be requested by your retinal specialist. The purpose of these tests is to confirm the diagnosis, evaluate the severity of the blockage, and possibly guide treatment. Fluorescein angiography (Figure 2, 4)may be employed in the diagnosis of retinal vein occlusion. In this procedure, a dye is injected into an arm vein and travels through the circulatory system to the eye. The dye acts as a tracer and is photographed as it passes through the ocular circulation. The site of blockage can often be clearly identified. Leakage of dye is also often seen from macular edema and areas of neovascularization. Areas of ischemia can be identified and measured.
Optical coherence tomography (OCT) is commonly used to obtain a high-resolution image of the retina and any associated macular edema (Figure 5). This technique is very valuable in quantifying the degree of fluid and monitoring response to potential treatment (Figure 6).
Treatment of retinal vein occlusion is aimed at macular edema and complications of neovascularization.
Macular edema can be treated in two ways. In BRVO, laser treatment has proved beneficial in reducing the retinal swelling and improving vision compared to no treatment; unfortunately, no such benefit was found for CRVO with laser in most cases. A newer technique for treating macular edema from both BRVO and CRVO involves an injection of medication directly into the vitreous cavity (see sections on Intravitreal Injection and New Medications). Although experimental, this new therapy has shown great promise in alleviating macular edema and restoring vision in many cases. Current medications being injected include a corticosteroid (Kenalog) and an anti-neovascular agent (Avastin).
Complications of neovascularization include bleeding from abnormal blood vessels and a severe form of glaucoma, called neovascular glaucoma. At the earliest signs of any of these complications, your retinal specialist will recommend a different laser treatment called panretinal photocoagulation to target the abnormal vessels and help prevent these complications. These complications have the potential of causing blindness, and can even result in the loss of an eye in the worst cases.